The invention relates to a silver catalyst and to a method for its preparation.
Solid, metallic catalysts are widely employed in a variety of industrial applications such as isomerization, hydrodimerization, hydrogenation, alkylation, oxidation and cyclization. For maximum efficiency, the metallic catalysts are advantageously prepared having a large catalytically active surface area per unit volume of catalyst.
In general, to obtain a desirably high catalytic surface area, the catalytically active component is prepared as finely divided as possible. Unfortunately, at the elevated temperatures normally employed in catalytic operations, most metallic catalysts sinter rapidly, therebvy reducing the catalytically active surface area.
In order to obtain the required thermal stability, the catalytically active component is generally applied to a sinter resistant, inert carrier. Although the inert carrier dilutes the catalyst, the thermal stability provided by the carrier renders the produced supported metal catalysts generally more stable than an unsupported metal catalyst.
It appears to be particularly difficult to apply silver or silver compounds extremely finely divided onto conventional carrier materials such as SiO.sub.2 and Al.sub.2 O.sub.3. For example, it is usual to impregnate the carrier with a solution of the catalytically active material, whereafter the liquid is removed by drying. After drying, the compound is converted into a metal by a thermal treatment, e.g., by gas phase reduction. Unfortunately, impregnation, drying and thermal treatment, using a silver compound, leads to a broad range of silver particle sizes. In addition, to relatively small particles, many very large particles are present.
It has therefore been proposed to deposit the silver from a solution onto the carrier which is suspended in the solution. Thus, in German Offenlegungsschrift No. 1,963,827, a method is proposed in which silver is deposited on a suspended carrier from a dissolved silver complex. This method leads to much better results than impregnation and drying. Silver particles having sizes down to 5 to 20 nm can be applied onto carriers, such as SiO.sub.2 or Al.sub.2 O.sub.3. The smaller of the above sizes is obtained at low loadings of the carrier with silver, whereas higher loadings lead to a larger minimum size. As the silver surface area per unit volume depends on both the size of the individual particles and the particle density, i.e., the silver loading, it is desirable to produce very small supported silver particles at higher loadings.
With other metals it is possible to deposit still smaller particles at high loadings onto carriers. Various metals of Group VIII of the Periodic Table, for example, can be applied to carrier materials as particles of 0.5-1 nm. The present state of art renders this impossible with silver particles at high silver loadings. A support highly loaded with very small silver particles provides a large silver surface area per unit volume and thus exhibits an elevated activity. Consequently, a reaction rate technically required can be obtained at relatively low temperatures. Since especially with oxidation reactions, a better selectivity can be expected at low temperatures, it is important to produce very small supported silver particles.
There is still another difficulty due to the fact that extremely small supported silver particles are not available for technically feasible catalytic reactions. Silver particles having dimensions of minimally 5 to 20 nm are unstable on the conventional carrier materials such as SiO.sub.2 and Al.sub.2 O.sub.3 at higher temperatures. Specifically, the dimension of the silver particles increases relatively rapidly at elevated temperature up to 100 nm or even more. This takes place, either owing to migration by silver particles such as across the carrier surface or by dissociation of silver atoms or ions of the particles, followed by migration to larger silver particles. This undesirable growth of silver particles also occurs, if the starting product comprises small silver particles homogeneously distributed throughout the carrier surface.
In view of the deficiencies of the prior art, a silver catalyst having stable, extremely small silver particles (e.g., about 3 nm) attached to a sinter resistant carrier material and which catalyst does not exhibit the above mentioned deficiencies is required.